The present invention relates generally to laser hardened materials and structures, and more particularly to a novel optical filter structure for selectively blocking laser radiation of predetermined wavelength while passing radiation of other wavelengths.
Optical switching devices comprising transition or switching materials which are transparent in one state but which transform to an opaque metallic state when heated to a characteristic transition temperature are well developed for applications such as optical filters, modulators, laser output couplers, and the like. These devices are generally characterized by a transition from substantial transparency below the characteristic transition temperature to substantial opacity above that temperature.
The present invention comprises an optical reflection filter which is substantially totally transmissive at substantially all wavelengths below a characteristic temperature, but which becomes reflective upon being heated to a ferroelectric phase transition temperature upon absorption of invasive radiation.
The invention comprises an optical filter structure including alternate layers of ferroelectric material and another suitable material which have substantially equal refractive indices in the unswitched state below the characteristic temperature, and which have in the switched state an optical thickness substantially equal to one-fourth the wavelength of the invasive radiation to be block by the filter. When the filter of the invention is heated to the characteristic temperature, the refractive index of the ferroelectric layers changes to a different value which produces in the filter classic quarter wave interference. A filter according to the invention may therefore be configured to allow an optical system to continuously receive an operational signal and simultaneously block invasive, potentially destructive, radiation.
It is therefore a principal object of the present invention to provide an improved optical filter.
It is a further object of the present invention to provide an optical filter for selectively blocking radiation of preselected wavelength while passing radiation of other wavelengths.
It is yet a further object of the invention to provide an optical filter which is transparent below a characteristic temperature but which switches to a reflective state upon being heated to the characteristic temperature.
It is yet another object to provide an optical filter having fast response time.
It is yet a further object of the invention to provide an optical filter having low absorption losses prior to switching.
It is yet another object of the invention to provide an optical filter which is independent of the wavelength of the impinging radiation prior to switching.
These and other distinguishing features and objects of the present invention will become apparent as the detailed description of certain representative embodiments thereof proceeds.